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Jeep Renegade Trailhawk – technical analysis

07 Dec 2015

Robert Pepper 8

The new Jeep Renegade Trailhawk is one of the most capable no-low-range 4X4s made today, so what’s the secret to its success?

This post complements our on/offroad review of the Trailhawk.

Compared to a standard Renegade the Trailhawk has:

• 30mm suspension lift, giving a handy 211mm of ground clearance, pretty close to what a medium-sized 4X4 with a live axle would have
• Full-size spare not space saver
• A recovery point at the rear, and on non-Australia models two at the front (for Jeep’s official comment on this see our full Renegade review )
• “Long travel” suspension with 170mm / 211mm front/rear flex
• A 4WD system
• Lower gearing for every one of the 9 speeds (higher final drive ratio)
• Bashplates
• Hill Descent Control
• All-terrain tyres
• Selec-Terrain
• Lots of badging making sure everyone knows this is a Trail Rated Jeep

Jeep told us:

The Trailhawk name is used to signify our most off-road capable vehicles. All Trailhawk vehicles are ‘Trail Rated’. There are no immediate plans to introduce any more Trailhawk models.

The exception to the Trailhawk range is the Wrangler Rubicon, which is an even more capable variant of the Wrangler equipped with front and rear locking differentials, swaybar disconnect, stronger transmission, very low crawlers gears and other good stuff.  I expect that as all Wranglers are sufficiently Trail Rated there is no need to apply special names to them.  Perhaps Jeep could signify the Rubicon’s extra capability by rebranding it “TrailWedgeTailedEagle” and “Australian Trail Rated”.

Anyway, let’s take a look at the Renegade Trailhawk tech in more detail.

4X4 Active low

Like most vehicles in its class, the Renegade uses an “on-demand” 4X4 system, in this case from British company GKN, which means it drives the front wheels and leaves the rears trailing.  This is done for fuel efficiency, not dissimilar to manual or auto locking hubs back in the days of part-time 4X4.

The rear wheels are only driven as and when the computer says so.  The Renegade Trailhawk has numerous sensors – one wheel speed sensor for each wheel, a yaw sensor, throttle, gear…possibly even a pitch sensor.  Either way, the car is very aware of what it’s doing and the computers crunch the numbers, then decide how much if any torque should be sent to the rear wheels.  There is no centre differential, it’s some sort of clutch pack built into the rear axle.

The general problem with this approach is that it biases torque to the front wheels which is not great for handling onroad, and leads to problems offroad such as the front wheels spinning and the rears sitting there on a tea break.  Then the centre clutch overheats, the entire car shuts down and you are going nowhere.  So in a situation like this where almost all the weight is over the rear the average softroader would simply fail to proceed.

But the Jeep Renegade Trailhawk is not your average softroader and indeed the car drove out of the above situation very slowly and under complete control.  This is because Jeep have added a “Lock” feature to the driveline which, they imply, locks the front and rear axles for a 50/50 torque distribution.  Well, that’s not actually true, but it does well enough that the rear wheels do something useful and don’t want (too long) for the fronts to spin before playing their part.  Also, the traction control (explained below) is wonderfully good.

The Lock button can be shown not to lock quite simply.  If it really did lock 50/50 front/rear then when you turn the vehicle on a high-traction surface you’d get a nasty graunching sound as the front and rear axles tried to travel at different speeds, yet were prevented from doing so.  This is called transmission windup, or “crow hop” in Jeep’s terms.  So the Lock button is a misnomer.  Also, I detected during testing some behaviour that indicates a front drive bias, specifically loss of traction on the front wheels before rear.  We fixed this by airing down the front tyres more than the rear.

The Selec-Terrain system should also change the torque bias front/rear, but I don’t have an details on how it does so.

I suspect that the torque bias is also changed when the steering wheel is turned so as to faciliate turning and avoid windup.  Therefore, as with other softroaders, a good technique is to do the hardest work in a straight line with maximum drive to the rear.  This also means less stress on the CV joints too so is a good general practice.

Another big test of any 4X4 system is when you drive slowly up a steep hill.  A poor system will have the car spinning the front wheels and the rears doing nothing, so the car fails the hill through engineering stupidity rather than loss of traction or clearance.  Then when you stop and apply the brakes the front wheels lock, the rears rotate, the front end slews around, the car rolls and life quickly becomes quite unhappy.  

Jeep has managed to avoid this and under test the Renegade did what it should do and that is fail by spinning front/rear diagonal wheels, leaving the other two wheels to provide lateral stability, and when backed down the driveline was locked enough that the front wheels didn’t lock up under braking, and we survived to write this result. Good work, Jeep.

Summary is that Jeep’s Active Low is a very good on-demand 4X4 system, and I’m prepared to say it’s the best on the market, but even so it’s not perfect and for serious offroad work I’d prefer a good old fashioned lockable centre diff.

Low range?

The Renegade has a 4X4 Low button which at first glance appears to engage a low range.

It does not.  The Renegade has no low range – there is no reference it to in any of Jeep’s documentation or websites, and I searched for a while.  There are two other obvious indications there is no low range:

1. The speeds the car does in 1st, 2nd and 3rd gear at 2000rpm are precisely the same whether Low is engaged or not.  Or any other gear, or any other rev range.
2. Jeep go on and on about the 20:1 crawl ratio.  Multiply the first gear ratio of 4.7 and the final drive of 4.334 and you get  20.37:1.  Therefore, there can be no further reduction gearing.  The final drive on non-Trailhawks is 3.734 so that is a crawl ratio of 17.55:1.  Still pretty good for a softroader.

UPDATE:  Some more information on this button direct from Jeep.  When pressed, the button:

• alters the throttle pedal sensivity
• alters the stability control calibration (explained below)
• force the car to start in 1st gear in Auto and Snow mode; usually it’d start in second.  Sand and mud mode would always launch in 1st gear, as does Rock.

So it is a mystery to me why Jeep bother, as different Selec-Terrain modes should modify the gear settings and stability control settings, not to mention engaging 4WD Lock.  Overall, I think this low button is just a marketing exercise.  You also have to stop the car to engage it, which is probably just for show given there is no actual changing of gears going on just a software change, but you can disengage at any speed. 

You can tell the car pulls away in second normally – for example come to a stop, flick the shifter over and it reveals the car is in second gear ready to pull away. 

By the way here are the rest of the gear ratios:

1st 4.70
2nd 2.84
3rd 1.91
4th 1.38
5th 1.00
6th 0.81
7th 0.70
8th 0.58
9th 0.48
Reverse 3.81

As you can see 5th is the 1:1 ratio, and as usual first is lower than reverse.

And some other crawl ratios: Cherokee Trailhawk 56:1,  Wrangler Rubicon 73:1.  Both of these vehicles have low range as understood by everyone else in the world for many years, since 1941 in fact.  The Cherokee does its low gearing a bit differently as it has its reduction gears in each differential rather than a separate transfer case, but it is still a set of lower gears regardless of how it is implemented.   The numbers for the Cherokee are 4.7 first gear, 2.92 for the reduction gears and 4.08 final drive so multiply them out = 55.9 and there’s your proper 56:1 crawl ratio.  The Wrangler has a conventional transfer case. 

The reason the Cherokee has its reduction gearing in-diff is to save weight and space, as well as make sharing the front-drive platform easier.  We can expect more of this, at least until the time when reduction gearing is dropped completely as electric/hybrid drive takes over.

Traction control aka Brake-lock differential system (BLDS):

Here again Jeep’s naming is misleading.  The “brake lock differential system” is nothing more than good old brake traction control – one wheel spins, it gets braked individually, torque is (in effect) sent to the wheel with traction, on you go. It is not a locking differential at all, and nor is it a limited-slip differential. 

It is however tremendously effective and is a big part of making the Renegade Trailhawk the brilliant offroader it is. 

For more information we have written a detailed explanation of traction control systems and stability control.  Please don’t get the two mixed up.  Electronics in 4X4s are here to stay so you’d best understand them.

The graphic below is again misleading..it says traction control is connected. What is means is stability control is connected.  Traction control is always connected.  See above for an explanation of the two.

Selec-Terrain

Back in 2005 Land Rover started the adaptive-terrain craze with Terrain Response.  Now everybody’s doing it, for better or for no effect at all.  Here is Selec-Terrain on the Renegade:

Briefly, adaptive terrain systems are modes you set the vehicle in to optimise it for a given terrain.  For hard, slippery surfaces like icy shallow snow you want a gentle throttle response, minimum wheelspin, and high gears.  For soft surfaces like sand or deep gravel you want brute power, hanging on to gears for longer.  And so on.

Here’s what Jeep have to offer.  The words below are Jeep’s, and my edits [ look like this ].

Auto:
The basic mode that’s calibrated for everyday driving. In most on-road conditions (dry and above freezing temperatures), the Auto mode disconnects the rear driveline to ensure optimal fuel efficiency. In the driveline disconnect function, virtually all elements behind the front transaxle (everything from the power transfer unit rearward including rings, pinions and the prop shaft) stop spinning to eliminate relating parasitic (to fuel economy) elements. However, when low co-efficient surfaces are detected (within milliseconds and unbeknownst to the driver), the system will automatically switch to the 4WD configuration (everything’s ‘re-connected’) to ensure the optimal traction required.

Snow:

The system converts to a specifically calibrated 4WD mode and is tuned (via chassis controls and transmission calibrations) to minimize oversteer and maximize traction. [ chassis controls = the stability control system, traction control and automatic gear shift points and start-off gear ]

Mud:

The system converts to a specifically calibrated 4WD mode to maximize low-speed traction on slick muddy surfaces by providing additional wheel slip (via specifically tuned chassis controls, differentials, and transmission ratios geared to a mud-slick surface). [ actually the transmission ratios won’t change, they mean selection of different gears ]

Sand:

The system converts to a specifically calibrated 4WD mode to maximize low-speed traction on looser, sand covered surfaces by providing additional wheel slip (via specifically tuned chassis controls, differentials, and transmission ratios geared to a sand/loose particle surface)

Rock:

The system includes 4WD low capability. This mode enables 20:1 “crawl” speeds/ratios for serious off road maneuvers and true, unmatched “Trail Rated” capability [ the 20:1 is already there, it isn’t enabled by Rock mode ]

We also believe the snow mode splits torque 60/40 front rear.  In some Jeeps that have the system a Sport mode also splits torque 40/60 front/rear.

Is it effective?  Read the roadtest.

Braking

Some standard 2015-era cleverness here.  The vehicle has ABS, which stops the wheels locking – contrary to popular opinion ABS doesn’t wait for a wheel to actually lock, it senses the wheel is slowing relative to the rest and momentarily relaxes brake pressure.  If you think about it, before a wheel locks it must slow down.  And the main function of ABS is to retain steering control under hard braking.
 
Now all you oldsters can stop rolling your eyes.  Modern ABS is good, and on the Jeep there is rough-road detection – the system can detect if the road surface is rough by the oscillations in the wheel speed signals.  Over a smooth surface a wheel will turn at a more or less constant speed, as the road gets rougher the wheel fractionally speeds up and slows its rotation.  An extreme example of this is when a wheel is lifted entirely in the air.  When the car detects it is on a rough surface it will brake such that there is additional “slip” which means slightly locking the wheel, not letting it rotate as freely as it would on a high-traction surface.  And as we all know, that means you stop quicker on a loose surface. 
 
Here it is in action:
 

 
Braking hard with a car like the Renegade is easy, just “cockroach” the pedal which means stamp on it hard as you can, and the electronics pretty much sort the car out.  Still, it also helps to look where you want to go and steer where you want to go, as is being done in the photo with a fraction of right lock as the rear is drifting a touch.

The Renegade also has EBD, which is Electronic Brake Distribution.  Plain old ABS just stops a wheel locking.  EBD sends braking force to the wheels that can best handle it.  Road cars are naturally brake-biased to the front wheels because that’s where the weight is transferred under braking, and in an emergency you’re best off locking the front wheels and ploughing straight rather than locking the rears and spinning off the road.  Unfortunately, this means the rear wheels have some unused braking capacity, and EBD puts that right by electronically distributing brake force around the four wheels.  

ABS and EBD are by no means unique to Jeep or the Renegade, but they have done a bit of work on what’s called a split-mu stop.  “Mu” is a term meaning coefficient of friction, where say a 1500kg car with a mu of 1 requires 1500kg to move it sideways.  Low-mu surfaces are ice, wet grass; high mu surfaces are dry bitumen, dry slick rock.  A hiliarious joke if you are a vehicle engineer – “two kittens on a roof, which one slides off first?  The one with the lowest mew”. Anyway, here’s a split-mu braking situation:

If a vehicle is braked hard and two side wheels are on a high-traction surface and two on a low-traction surface then you are very likely to spin the vehicle and perhaps roll it as the low-traction wheels lock and slide, and the high-traction wheels slow the vehicle.  Of course this is theory but I have proved it in practice…let’s just say it’s exciting.  The photo above shows a very slight clockwise yaw and counter-steer which is what you’d expect with the two left wheels on dirt. 

What Jeep have done is design the ABS/EBD/ESC systems so that this situation is recognised and appropriate braking force is sent to each wheel to keep things straight. The electric power steering – EPS, or sometimes EPAS (“assisted”) – also controls the steering wheel to help keep the car straight. Yes, less work for the driver to do, less skill required.  It’s the way things are going, like it or not.

Electric Power Assisted Steering (EPAS)

So why is electric power assisted steering becoming so popular?  Safety and efficiency are the two main answers.  Compared to a conventional hydrualic power steering system an electric saves around 3% of fuel, and every little bit counts.  An electric system can also be easily controlled by the car itself, which is how self-parking systems work, and the split-mu system above. 

The Lane Depature system also works off the EPAS – after the car figures out you’re about to cross a white line then the EPAS will turn the steering wheel a bit to help keep you straight.  It can even detect whether or not you are holding the steering wheel by the presence of absence of tiny torque inputs from the driver as they hold the wheel. In the case of the Renegade there is a beep and a warning to tell you to get your hands back on the wheel!

Hill descent control

Here’s Jeep’s words:

Hill Descent

Enables a smooth and controlled hill descent in rough terrain without the driver needing to touch the brake pedal. When on, the vehicle will descend serious grades using the ABS brake system to control each wheel’s speed to ensure optimal pace and momentum.

The system works simply – you switch it on, don’t touch the pedals and the car maintains a set speed down a hill, braking individual wheels.  It works down to 2km/h and is highly effective.  If you want to vary speed that’s simple, just use brake or accelerator to slow or speed up, and when you get back off the pedals the car does the rest.  Works very well indeed on the Renegade, and even better on the Grand Cherokee which has the advantage of lower gears because it has an actual low range.

Koni FSD shocks

The Renegade runs Koni shocks (dampers to UK and other readers), specifically the FSD models – that’s Frequency Selective Damping. Fancy name for a simple principle.  A typical shock has a piston which move up and down within an oil-filled cylinder, not dissimilar to a coffee plunger.  Oil is forced through a series of valves and shims and that controls the compression and rebound of the shock.  The FSD system works the same way, but there’s a bypass, a second way for the oil to flow as the piston moves up and down.  The FSD system closes this bypass off and allows pressure to build, then opens it.  It’s an entirely mechanical system so no sensors or computer control. 

Koni make all the usual claims about brilliant handling and the like as is usual with every suspension innovation.  Does this design actually make a difference in reality?  Maybe, maybe not…I’d suggest that the skill of the suspension tuner and the inherent quality of the shock plus chassis design has more to do with it than this bypass.  Regardless, the suspension in the Renegade works very well offroad, there’s not much flex but the damping and tune in general is well done.  Here’s an example:

There’s a tech explanation here from Koni themselves:

And all of the above adds up to this:

Here’s a couple of shots courtesy of Barlow Jeep School.  Anyone recongise the racetrack?